Remove some unnecessary code, including:

 - Explicit zeroing of memory allocated for dmam_alloc_coherent()

 - Some duplicated code

 - Some redundant masking
Signed-off-by: Xiang Chen <chenxiang66@hisilicon.com>
Signed-off-by: John Garry <john.garry@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>